There have been significant advances in the sensing industry based on requirements for such fields as airport security as well as military and medical applications. For example, it is broadly publicized that the airport and ship terminal screening measures for explosives, radioactive or biological dangers have been hampered by inadequate sensing equipment.
There are various types of sensors and sensing applications. Current gas sensors are typically based on mass loading of a sensing film upon exposure to a target analyte. Mass loading refers to measuring changes of the vibrating member due to on an increase of the mass caused by an adsorption of some gas. A mass loaded resonator has electrodes and the device vibrates at some resonant frequency. As the gas molecules are adsorbed by the sensing film, the added mass of the gas molecules causes a change in the propagation or resonance of the acoustic wave device (AWD). For such a device the resulting change is a frequency decrease.
Another sensing area involves cantilevers. One cantilever method employs a highly sensitive cantilever structure with optical detection based on bending of the cantilever. A further cantilever technique is a piezoresistive/piezoelectric element on the cantilever that directly senses induced strain caused by bending of the cantilever as a resistance or voltage change. However, as detailed herein, these prior systems fell far short of the customer requirements in the expanding array of sensing applications.
Some examples of the current art are described herein in general terms for illustrative purposes. Referring to FIG. 1a, which is a capacitance-based platform disposed in a package, typically a microelectromechanical (MEMS) capacitive diaphragm sensor. Disposed upon the substrate 5, there is a lower plate 20 and a vented upper plate 10 with a central fill hole 25. A polymeric sensing film 30 is disposed between the lower plate 20 and the upper plate 10. Electrical connectivity is provided by the lower plate electrical connector 15 that allows the energy source and response measurement connections. Typically these devices use thick film polymer sensing films to form a sensor array. As known to those in the art, there is a change in the dielectric constant of polymers upon exposure, thereby allowing detection. In this capacitive sensor design, there generally is a consistent gap width between the plates and a good baseline. Such a system is typically low in power and there is no pre-concentrator.
FIG. 1b depicts a SiC resonator which typically is constructed in a sensor array, such as 2-5 microns, and uses mass loading for detection. The piezoelectric low frequency MEMS structure in FIG. 1b illustrates a P-type silicon substrate 40 upon which is disposed an N-type material 45, such as SiC. Not only is the substrate 40 covered by the N-type material 45, but the N-type material 45 extends outwardly from an end of the substrate 40 creating an extended type structure which shall be termed a cantilevered beam. An epitaxial piezoelectric layer (AlN) 50 is disposed on portions of the P-type silicon substrate 40, including portions on the cantilevered beam. An upper electrode 55 is disposed upon the epitaxial piezoelectric layer 50. A lower electrode contact 35 is coupled to the N-type layer on a portion above the substrate 40 without extending onto the cantilevered beam. This structure may include a pre-concentrator to increase sensitivity, but takes further time to collect samples, such that they generally do not operate in real time. This typically uses thick film polymers and subject to polymer thickness control issues. The resonator is more prone to Q and temperature stability issues. It is also not energy efficient with power spikes for required heating of the pre-concentrator. For illustrative purposes, a further description of this general type of device can be found in U.S. Pat. No. 6,953,977.
FIG. 2a illustrates a surface acoustic wave (SAW) delay line structure commonly used for sensing applications. This two port structure serves as a gas sensor by placing a gas specific sensing film 65 on the surface of the device in-between an input transducer 60 and an output transducer 70 which is disposed upon a piezoelectric SAW substrate 75. When the sensing film 65 gets exposed to a gas, mechanical and electrical perturbations of the sensing film 65 causes a corresponding change in the propagation characteristics of the AWD. When the SAW structure is coupled to an oscillator circuit, the result in an increase or decrease in oscillator frequency. Some examples of sensing films can be metal, metal oxide, metal nitride, metal carbide, polymer, fluoropolymer, silane, siloxane, silicone, or biological material (antigens, bacterial biofilms, biopolymers, or cell cultures). In certain combinations of films and device structures it is possible to extend the sensing film over the entire device or to employ the device conductors or substrate as the sensing medium. In addition to delay lines SAW resonators also exist. Prior SAW resonators include those described in U.S. Pat. No. 6,335,667 and the multi-reflective AWDs as disclosed in U.S. Pat. No. 7,002,281. There are numerous other waves related to the SAW in their use of similar transducers, called surface generated acoustic wave (SGAW), and these terms shall be deemed interchangeable.
The SGAW-based platforms such as those illustrated in FIG. 2a typically have thick film polymers forming the sensor array. The sensing film is in the delay path and employs mass loading to detect change in frequency. A pre-concentrator can be used to increase sensitivity however the heating can cause high power spikes and is less energy efficient.
There are also known systems that measures displacement of a cantilever using optics. Referring to FIG. 2b, a MEMS cantilever system with optical detection is depicted in which the mechanical resonance of a mechanical structure is employed. This type of system generally employs a silicon substrate which is not a piezoelectric material. The silicon substrate 80 has a cavity portion 100 and the structure includes layers about the periphery of the device, including a cantilever 85 extending from an end of the device. There is a sensing film 95 disposed upon a film surface 90 wherein the film surface 90 relates to attachment chemistry for disposing a sensing film 95, which is typically a metallization layer.
The silicon MEMS cantilever assembly 85 is typically coupled with a piezoelectric transducer or electrostatic driver (not shown) that is driven with an oscillator (not shown) to vibrate at the mechanical resonance of the cantilever 85. The laser diode 105 emits a laser beam signal 110 that reflects off the cantilever surface to the detector 115. The measured response can be used to lock that relative frequency and it can detect changes from that relative frequency. Alternatively, it may be able to measure the difference between the frequency of the driving oscillator and the actual measured frequency.
Induced bending of the cantilever is in resonance mode and it uses a bi-cell detector 110 that measures the frequency and may also measure bending effects for resolution. This optical cantilever system measures the change of the angle of deflection of laser light 110 typically from a laser diode 105. The laser light 110 incident upon the end of the cantilever 85 typically bounces off the end of the cantilever and is received at an optical detector 115 such as a bi-cell optical detector. As gas affects the sensing film 95, there is a change to the cantilever 85, and the optical detection 115 measures changes to the angle of deflection of the laser light 110 bounced off the end of the cantilever 85. While complicated, the stress induced effects on the cantilever 85 combined with the optical detection provides satisfactory sensitivity. Nonetheless a simpler, more robust system is desired, as is better sensitivity and resolution.
In sum, sensors of this type typically measure changes in bending of the cantilever or changes in resonant frequency of the mechanical structure. A change in the bending or strain of the sensing film translates into a direct change in the shape of the cantilever which is then measured by the optic detection system. Another approach uses changes in film mass or stiffness of sensing film to affect the overall spring mass constant of the cantilever altering its resonant frequency. In this resonance system, the cantilever needs a piezoelectric transducer and oscillator or electrostatic means to drive the unit in addition to the optical detection mechanism. Sensors of this type are controlled by the exact shape and resonant frequency of the cantilever itself, limiting the design and implementation and incurring direct damping of the cantilever resonant Q by the sensing film. For illustrative purposes, U.S. Pat. Nos. 5,719,324 and 5,445,008 describe background information for sensors of this type.
While sensing platforms are available, the industry demands a sensing technology that has a significant improvement in sensitivity and simplicity over existing sensing technologies.